1. Field
At least one embodiment of the present invention generally relates to a curvature correction device that corrects a curvature of a main scanning line in an optical scanning unit that optically scans a plurality of scanning targets, an optical scanning unit using the curvature correction device, and/or an image forming apparatus using the optical scanning unit.
2. Description of the Background
Image forming apparatuses are used as copiers, facsimile machines, printers, and multi-functional devices combining several of the foregoing capabilities. Certain types of image forming apparatuses such as laser-beam printers, digital copiers, and laser facsimile machines optically scan a latent-image bearing member such as a photoconductor with a light beam generated based on image information to form a latent image on the latent-image bearing member. Such image forming apparatuses typically have an optical scanning unit including a light source such as a laser diode, a deflection unit including a polygon mirror, an fθ lens, and a reflection mirror. The light beam emitted from the light source is deflected by the deflection unit in a main scanning direction of the optical scanning unit, converged by the fθ lens, and reflected by the reflection mirror toward a surface of the latent-image bearing member.
Such optical scanning unit may have a subtle distortion due to manufacturing errors of its optical components and/or support members. Alternatively, such optical components and support members may be deformed by heat of a motor during optical scanning operation. Further, such optical components and support members may have assembling errors. Such distortion, heat deformation, and/or assembling errors may curve a main scanning line on the surface of the latent image bearing member, thereby preventing a desired image from being normally produced.
Such curvature of the main scanning line may be resulted from the accumulation of tolerances including a structural distortion, heat deformation, and/or assembling errors of components of the optical scanning unit. As a result, the amount or direction of curvature may vary depending on products of the optical scanning unit. In order to perform optical scanning operation properly, it is necessary to correct such curvature of main scanning line on the surface of the latent-image bearing member even when the main scanning line is curved in a direction toward any side of upstream and downstream sides with respect to a sub-scanning direction of the optical scanning unit, that is, a moving direction of the surface of the latent-image bearing member.
In view of the above-described situation, Japanese Patent Application Laid-Open No. 2006-17881 proposes an optical scanning unit. The optical scanning unit has a curvature correction device capable of correcting the curvature of main scanning line even if the main scanning line is curved toward any side of upstream and downstream sides with respect to the sub-scanning direction. The curvature correction device uses a holder to hold a reflection mirror in such state that the reflection mirror is forcibly curved or bent in a thickness direction of the reflection mirror. The curvature correction device also uses an adjuster of a pushing member to push the reflection mirror held in such state, thereby providing a force for bending the reflection mirror in a direction opposite to the direction in which the reflection mirror is forcibly bent by the holder. In such configuration, a slight push of the reflection mirror by the pushing member can reduce the amount at which the reflection mirror is forcibly bent by the holder.
A further push of the reflection mirror may bend the reflection mirror in a direction opposite the direction in which the reflection mirror is bent at the initial state. Thus, such curvature correction device capable of bending the reflection mirror toward any of the upstream and downstream sides may correct the curvature of the main scanning line by countering the curvature of the main scanning line at the initial stated with the bend of the reflection mirror in the direction opposite the forcibly bent direction.
However, when the inventors of the present disclosure built a prototype of such curvature correction device and actually mounted on an image forming apparatus, such curvature of main scanning line was preferably corrected at an initial period, but was not properly corrected over time. According to the examination of the inventors of the present disclosure, one possible cause was found that the holder holding the reflection mirror in the forcibly bent state was gradually deformed by the resilience of the reflection mirror.